Transition metal catalysts such as palladium and titanium catalysts are extensively used in various industrial processes including the hydrogenation of alkenes and alkynes, the oxidation of alcohols, the olefination of alkyl halides, and carbon-carbon coupling reactions.
Homogeneous transition metal catalysts are difficult to separate for reuse, and therefore, there have been various attempts to develop recyclable transition metal catalysts, e.g., heterogeneous palladium catalysts obtained by immobilizing palladium moieties onto inorganic supports or organic polymers; palladium nanoparticles; colloidal palladium species; and polymer-incarcerated palladiums ([Djakovitch, L. et al., J. Am. Chem. Soc. 2001, 123, 5990; Kim, S.-W. et al., J. Am. Chem. Soc. 2002, 124, 7642; Roucoux, A. et al. Chem. Rev. 2002, 102, 3757; and Akiyama, R. et al., J. Am. Chem. Soc. 2003, 125, 3412]).
The conventional recyclable catalysts, however, suffer from the problems of low reactivity, deactivation, leaching of the metal component and complicated synthetic procedures.